The survey on takes place on the west coast of the North Island. This same approach has been used for estimating some Hector’s dolphin sub-populations and is ideal for assessing small populations.
This work is a key aspect of our programme to protect Māui dolphins. It involves analysing DNA profiles of individual dolphins taken from biopsy samples obtained from the animals during dedicated small-boat surveys. The surveys go as far north as the entrance of Kaipara Harbour and as far south as the Mokau River, Taranaki, although most effort is concentrated between Raglan Harbour and Muriwai.
The DNA profiles are collected by firing a small, lightweight biopsy dart from a modified veterinary capture rifle into live free-swimming dolphins from the survey boat. The biopsy dart hits and bounces off an animal removing a tiny plug of skin and associated blubber. The samples collected are relatively small, only about 4 mm by 10 mm. Calves, distinguishable by their small size, are excluded from sampling.
This method of sampling provides a tiny genetic sample from which it is possible to derive vital information about the dolphins in the survey area such as:
- The ‘effective population size’ (estimates the effective number of breeding adults in the parental generation).
- An indication of the genetic diversity within the population.
- An estimate of how many are males and how many are females in the population.
- How much individual animals move around over both the short-term (i.e. days) and long-term (months/years).
- An understanding of the whakapapa of individual animals.
This method has been successful in 2011, 2016 and 2021 to derive abundance estimates for the Māui dolphin population. We’re confident the methodology is robust and follows best practice. Using uniquely identifiable individuals to estimate the size of a population has been a standard method in marine mammal science since the first publication in the late 1970s recognising that photos of fins could be used to uniquely identify bottlenose dolphins.
The most recent Māui dolphin abundance survey was completed in 2021, and you can find the results of that survey on this page.
2020-21 abundance estimate
In 2020 DOC and Ministry for Primary Industries, in collaboration with Auckland and Oregon State Universities, began a repeat of the abundance estimate of Maui dolphins.
The revised population estimate was carried out over two summer seasons, 2020 and 2021. The 2020 and 2021 field reports are available below.
Key findings
- The abundance of Maui dolphins over 1 year of age was estimated to be 54 with a 95% confidence interval of 48 to 66 (this means that the researchers are 95% confident that there are between 48-66 Maui dolphins over the age of 1 year within the survey area).
- Over both years a total of 84 biopsy samples were collected from a total of 43 individual dolphins.
- 22 of these individuals were females
- 13 individuals were sampled in both years
- 41 individuals were identified as Maui dolphins
- 2 individuals (one male and one female) were identified as Hector’s dolphins, both sampled in 2020 and not resampled in 2021. The female Hector’s dolphin was first identified in 2010, while the male had not been sampled previously.
The 2020 and 2021 field seasons were successful in matching the effort of the 2010/2011 surveys and the 2015/2016 surveys, with a comparable number of surveys, duration of the survey period and coverage of the primary known habitat for Maui dolphins.
Reports
Estimating the abundance and effective population size of Māui dolphins in 2020 – 2021 using microsatellite genotypes, with retrospective matching to 2001:
Māui dolphin biopsy sampling survey field reports and preliminary results:
Previous abundance estimates
2015-16 abundance estimate
In 2015 DOC and Ministry for Primary Industries, in collaboration with Auckland and Oregon State Universities, began a repeat of the abundance estimate of Māui dolphins.
The revised population estimate was carried out over two summer seasons, 2015 and 2016. The 2015 and 2016 field reports are available below.
Key findings
- The abundance of Māui dolphins over 1 year of age was estimated to be 63 with a 95% confidence interval of 57 to 75 (this means that the researchers are 95% confident that there are between 57-75 Māui dolphins over the age of 1 year)
- Over both years a total of 92 biopsy samples were collected from a total of 51 individual dolphins
- 32 of these individuals were females
- 17 individuals were sampled in both years
- 49 individuals were identified as Māui dolphins
- 2 individuals were identified as Hector’s dolphins, one of which was sampled in the previous surveys (2010/2011), and a male Hector’s was sampled in both 2015 and 2016.
The 2015 and 2016 field seasons were successful in matching the effort of the 2010 and 2011 surveys with a comparable number of surveys, duration of the survey period and coverage of the primary known habitat for Māui dolphins.
The results of this abundance estimate will be used to inform the review of the Hector’s and Māui dolphin Threat Management Plan in 2018.
Reports
Estimating the abundance and effective population size of Māui dolphins using microsatellite genotypes in 2015-16, with retrospective matching to 2001-16:
Māui dolphin biopsy sampling survey field reports and preliminary results:
2010-2011 abundance estimate
In 2010, DOC began field work in collaboration with the University of Auckland and Oregon State University to obtain a new abundance estimate.
During two austral summers (from February to March) in 2010 and 2011, the research team collected samples from a total of 41 different individuals. Some individuals were sampled more than once and by examining how many of the same dolphins were resampled in the second round of research against the number of new individuals sampled, a revised abundance estimate was calculated.
Key findings
- The abundance of Māui dolphin of over 1 year of age for 2010-11 was estimated to be approximately 55 with a 95% confidence interval of 48 to 69 (this means that the researchers are 95% confident that there are between 48-69 Māui dolphin over the age of 1).
- Utilising genetic samples from Māui dolphin available from 2001-07 and the current 2010-11 sampling period the rate of change for the population from 2001-11 could be estimated. The result suggests a slow decline over the past decade (-3% per year), although this trend could not be confirmed with 95% confidence.
- In 2010, DNA fingerprinting of the samples collected showed that there were two female South Island Hector's dolphins swimming with Māui dolphin. In 2011, one of those Hector's dolphins was sampled again in a similar area. This is the first record of this occurring.
- Although there is no evidence that these female Hector’s dolphins have interbred with Māui dolphin, it is possible that they can. This could enhance the genetic diversity of the Māui dolphin population.
- An individual dolphin moved about 80km in less than three weeks, with others moving in the vicinity of 30km before being sampled again. This indicates that the individuals remaining in the population can still mix amongst themselves and are not restricted to small isolated groups only.
- It seems that there are at least as many females as males in the population. Of the 41 individuals sampled, 25 were female and 16 were male. Having more females in a population usually gives the best chance of the species recovering.
Report
The data from this report is not directly comparable with earlier aerial surveys (see 2006 estimate) because of the different methods used, but both the reports highlight that the population is very small and are indicative of a recent decline.
2006 estimate
The abundance estimate published in 2006 was made from extensive aerial surveys by Otago University in 2004. The researchers then used a statistical model to come up with the estimate. This suggested that there were around 111 dolphins left (with a 95% confidence interval of 48-252).
Report
Other abundance estimates
- The Scientific Committee of the International Whaling Commission (IWC) endorsed two abundance estimates for the Māui dolphin population (for the population size in 2001 and 2016) at their 2018 meeting.
- The estimates were produced by Cooke et al. using the same data used by Baker et al. for their 2016 estimate
- Cooke et al. used a more sophisticated population model to estimate the trend in abundance and to account for likely mortality over time.
- The number has not gone down from 63 to 57. The lower estimate produced by the Cooke et al. model for 2016 does not represent a decline in the Māui dolphin population from the Baker et al. estimate but is simply a reinterpretation of the existing data.
- The two estimates by Cooke et al. and the estimate by Baker et al. have been endorsed by the IWC Scientific Committee as suitable for management purposes.
Key findings
- There was estimated to be 85 Māui dolphins over 1 year of age in 2001, with a 95% confidence interval of 54 to 133.
- There was estimated to be 57 Māui dolphins over 1 year of age in 2016 with a 95% confidence interval of 44 to 75.
- The model estimated with approximately 95% probability that the population declined over the period 2001-16.
- This model can be used to make projections under different mortality scenarios to estimate the likelihood of recovery under different levels of protection.